A recording medium such as a recording sheet or OHP sheet is usually placed in a positioned state by contacting with a wall face such as an outer peripheral wall of a sheet feed tray for an image forming apparatus.
In a sheet feed tray capable of placing recording media of various sizes has a wall-shaped regulating wall member, which is made movable to contact with an end portion of the recording medium thereby to position it, and a lock mechanism for regulating a displacement of the regulating wall member (see, for example, JP-A-1-209227).
The lock mechanism disclosed in JP-A-1-209227 includes a saw-tooth shaped engaging portion (or a corrugated portion) formed in the sheet feed tray, an engagement pawl disposed at the regulating wall member for engaging with the engaging portion and a coil spring for pushing the engagement pawl onto the engaging portion. The engagement pawl is made movable relative to the engaging portion.
When the position of the regulating wall member is to be changed, the engagement pawl is displaced away from the engaging portion to release an engaging state (or a locked state) between the engaging portion and the engagement pawl, and the regulating wall member is moved while holding that released state. Then, the engagement pawl is brought into engagement with the engaging portion and fixes (or locks) the position of the regulating wall member.